Vector modulation refers to a modulation technique in which the modulated signal can have arbitrary phase and amplitude relationships. Vector modulation is desirable in many instances, one of which is the transmission of digital information on RF carriers. One popular example of a vector modulator is an "I-Q modulator." In an I-Q modulator, the modulation signal is divided into an in-phase (I) and a quadrature (Q) component. These form the basis of two orthogonal signals that can be added together to form any combination of magnitude and phase. It is thus theoretically possible to use an I-Q modulator to generate any form of amplitude, phase or frequency modulation, or any combination thereof.
FIG. 1 illustrates a conventional I-Q modulator 10 which includes an L.O. source 12, a 90 degree phase shift network 14, first and second mixers 16a, 16b, and a summing network 18. The mixers are used as amplitude modulators. A first input modulation signal I is mixed by the first mixer 16a with the L.O. signal. A second modulation signal Q is mixed by the second mixer 16b with the L.O. signal shifted by 90 degrees. The two amplitude modulated signals are then summed together by network 18, resulting in an output signal whose amplitude and phase modulation can be described as the vector sum of the "in phase" signal I and "quadrature phase" signal Q. A mathematical description follows. Let the L.O. signal be given by: EQU V.sub.LO =e.sup.jwt ( 1) EQU V.sub.1 =I e.sup.jwt ( 2) EQU V.sub.2 =Q e.sup.j(wt+.pi./2) ( 3)
The output signal can then be given by: EQU V.sub.1 +V.sub.2 =(I+jQ)e.sup.jwt ( 4)
It will be recognized that the circuit of FIG. 1 can produce an output signal having an arbitrary amplitude and an arbitrary phase relationship with the L.O. signal. If I and Q are sinusoidal signals whose phases differ by 90 degrees, one of the sidebands generated by the upper mixer is cancelled by a sideband generated in the lower mixer. When used in this way, the circuit operates as a single-sideband mixer.
A drawback to the illustrated system is that phase and amplitude accuracy must be maintained in both signal paths. However, it is sometimes desirable to process these signals using circuitry that alters these characteristics. Such processing interferes with the resulting modulation. An example of such processing is frequency translation of the two component signals in a tracking filter that uses phase locked loops (PLLs). In such circuits, the phase locked loops generally remove the amplitude modulation from the component signals, making this frequency translation method only useful when the signals have constant amplitude. This is generally not the case.
In accordance with the present invention, this and other drawbacks of prior art vector modulation techniques are overcome by combining constant amplitude signals from phase modulators, rather than amplitude modulators or mixers, in producing the vector modulated output signal. Since the two component phase modulated signals have no amplitude modulation, they can be processed without regard to effect on the signals' amplitudes. Any change in their relative amplitudes can be corrected by a level control circuit prior to summing. This flexibility permits implementation of circuits, such as PLL-based tracking filters, that could not be realized using prior art vector modulation techniques.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.